dont throw away blown output transformers.

Discussion in 'The Workbench' started by MickeyJ, Jan 18, 2020.

  1. MickeyJ

    MickeyJ Well-Known Member

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    I just repaired an old fender twin? output transformer.
    A customer gave this to me years ago, I've used it as a paperweight but today I thought I'd take a peek at it.
    So one side of the primary was dead, nothing,zero resistance.
    i thought i'd open it up just in case.
    After cutting the paper winding coating I found the short. The winding, about 3cms from where the winding is attached to the primary wires, had shorted to the case, the silver case on the outside. Quite an arc, it's probably 6-7mm from the winding to the case in that location.
    About 1 cm of winding had converted into fire, leaving soot traces on the case and on that area of the core.
    I should have taken an image then but I got too excited and fixed it nice and quick.
    If you take a read good look at the first image, you can actually see the stub of the winding going back into the core wrap, just above the solder work. If you take a look at the last image you can see just how thin the primary winding of this transformer is, real thin.
    So now the primaries measure as they should, about 38 ohms on one side and 39 ohms on the other .
    I'll clean the rest of the soot off carefully and coat the area in nice thick epoxy to discourage any further shorts either to the case, or to the laminations.
    mail.google-1.jpg mail.google.jpg mail.google-2.jpg
     
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  2. South Park

    South Park Well-Known Member

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    The wire looks like it is bear wire but it has a coating on it to insulate it. Cut it back you should be good .
     
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  3. MickeyJ

    MickeyJ Well-Known Member

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    yeah i cut it back, no easy feat as its so thin, one heavy handed move there and it was all over south park, i just scraped along the remainder of the wire for about 5mm , top and bottom.
    After look at that first image, it does appear that it actually shorted to the lamination core, not the cover.
     
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  4. South Park

    South Park Well-Known Member

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    Yes it would be a stretch if it worked. It is amazing how transformers work. How all that wire does not short out. The transformers are big part of the tube sound.
     
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  5. Matthews Guitars

    Matthews Guitars Well-Known Member

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    For any amp that has some value, if a transformer fails, consider that it can be rewound by various transformer companies, Mercury Magnetics being the first one that comes to my mind. A rewound original transformer may not have the value of a pristine original, but it's more valuable than a modern copy.
     
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  6. MickeyJ

    MickeyJ Well-Known Member

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    if a transformer fails, and you can fix it, why get it re-wound?
    Problem with getting them re-wound is that its a bit of a crap-shoot tonewise, winding patterns change the sound of them.
    This one works real good after being fixed, no issues, no sparking.
    Admittedly this one was easy to fix, but the point of this thread is that they can be fixed sometimes.
    Wire is wire, as long as there is a connection, it's all good.
     
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  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Yeah well....once it arcs there is a carbon trace and it will keep burning.
    There is really no way to clean it off.
    I think that tranny is junk.
     
  8. MickeyJ

    MickeyJ Well-Known Member

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    ampmadscientist, try and be live up to your name a bit more.
    the wire is coated with insulation.
    these xformers aren't potted like early 60's one ( it seems this may be a good idea)
    there has been an arc out, but that new section of wire and all the wire close to it now has a 6-7 mm coating of 2 pac epoxy, hard pinky looking stuff.
    6-7 mm of epoxy vs 3 thou of acrylic/enamel insulation is no contest.
    The transformer has been tested in circuit and dimed, the transformer is as good as any its age.
    Sounds great actually, theyre not exactly hi-fi but they saturate ok and sound nice and old.
     
    Last edited: Jan 19, 2020
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  9. Micky

    Micky Well-Known Member VIP Member

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    Burning a wire like that on an OT is sometimes a result of a speaker impedance mis-match.

    When you don't match the speakers to your amp, you cause excess current to build up heat in the output transformer. Look at how small the wire is, and then imagine trying to push 2 or 3 times (depending on the severity of the mis-match) the amount of current thru it.

    Not saying this is what happened here, but the result is exactly as I have observed in the past when someone drives an amp too hard thru the wrong tap on an OT. In this case you got lucky where the repair was relatively close to the outside of the windings. Sometimes it happens deep inside and the xfmr is toast. Same thing with power transformers.
     
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  10. pedecamp

    pedecamp Well-Known Member

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    I always thought transformers could be repaired, a blown transformer from a vintage amp could be dissected and put back to working order by a transformer company. Yes?
     
  11. Micky

    Micky Well-Known Member VIP Member

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    Yes, but many times re-winding costs twice that of a new replacement.
    But for a priceless vintage amp it may be well worth it.
     
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  12. pedecamp

    pedecamp Well-Known Member

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    I thought I saw transformers going for $50-$80 bucks so to double that cost for a rewind is well worth it even on a newer amp you want to keep original. :yesway:
     
  13. Exojam

    Exojam Well-Known Member

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    MickyJ,

    I apologize but is there anyway you could make those pictures larger? My eyes are having a hard time seeing what everyone else is.

    Thanks.
     
  14. MickeyJ

    MickeyJ Well-Known Member

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    im re-sizing photos now
     
  15. MickeyJ

    MickeyJ Well-Known Member

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    This O/t was from a 1974 fender twin or showman if they still made showmans then.
    You can see in the hole for the screws that hold the covers on, this is what cheap is. There are no plastic or paper tubes that cover the threads for these long screws, the holes are slightly too small and the threads have chewed the laminations, this is not good, this will make the laminations conduct, there will be eddy currents and added heat until some corrosion sets in, or, if the screws stay in contact with the chewed out lamination tunnel hole, a longer time than that.


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  16. Exojam

    Exojam Well-Known Member

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    Thank you very much!

    Very interesting stuff.
     
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